The invention relates generally to fan drive systems and more specifically to a hydraulic fan drive system employing binary control strategy.
The present invention relates to friction coupling devices and fluid coupling devices, such as friction clutch assemblies and viscous drives, the fluid coupling devices being of the type that include both a fluid operating chamber and a fluid reservoir chamber, and valving to control the quantity of fluid in the operating chamber.
Although the present invention may be used advantageously in various configurations and applications, it is especially advantageous in a coupling device used to drive a radiator cooling fan of an internal combustion engine for an over-the-road truck, such as a class 8 truck, and will be described in connection therewith.
Friction coupling devices and fluid coupling devices that drive radiator-cooling fans generally comprise dry friction clutch assemblies and viscous drive clutch assemblies, respectively.
Dry friction clutch assemblies tend to have tow operating conditions “ON and OFF” referring to when a friction clutch is either fully engaged or fully disengaged. When a friction clutch assembly is providing cooling, the clutch is fully engaged and not slipping. When the friction clutch assembly is not providing cooling, the assembly is fully disengaged and slip speed is at a maximum between a clutch plate and an engagement surface.
The dry friction clutch assemblies generally have low thermal capacity, since they typically do not incorporate fluid flow cooling mechanisms. Thus, the clutch assemblies have minimal cooling capability and are unable to cycle repeat in short durations of time. Also, because of low thermal capacity, the clutch assemblies are also limited in torsional capacity, such that they are incapable of engaging at high engine revolutions per minute (rpm) or high engine speeds. The thermal energy that is generated during engagement at high engine rpm speeds can “burn up” or cause the clutch assembly to become inoperative.
Viscous drive clutch assemblies, on the other hand, have become popular due to their ability to cycle repeat, engage at higher engine speeds, and have varying degrees of engagement. Viscous drives have an operating range of engagement, are generally less engaged at higher engine speeds and are generally more engaged at lower engine speeds. Viscous drives are never fully engaged for internal viscous shear purposes.
Unfortunately, viscous drive clutch assemblies are also thermally and torsionally limited. Viscous drives slip to some degree at all times making them to be incapable of turning at fully engaged peak operating speeds or at higher speeds than originally designed. Since viscous drives are continuously slipping, they are continuously generating heat, unlike friction clutch assemblies. Viscous drives are further limited in that the more engine cooling that is needed, the larger and more costly the viscous drive and cooling fan that are required. Thus, for increased engine cooling requirements, viscous drive clutch assemblies can become impractical in size and cost.
Due to increased engine cooling requirements, a current desire exists for a fan drive system that is capable of not only providing an increased amount of cooling over traditional fan drive systems, but also that it have the associated advantages of a friction clutch assembly as well as viscous drive clutch assemblies, without the associated disadvantages. It is also desirable that the fan drive system be practical and reasonable in size and cost, so as to be approximately similar to and preferably not to exceed that of traditional fan drive systems.
To address these issues, a new system and method for engaging a fan drive was developed. This system, which is described in U.S. patent application Ser. No. 10/624,070 filed Jul. 21, 2003 entitled “Hydraulic Controlled Fan Clutch with Intergral Cooling”, to Robb et al, which is herein incorporated by reference, discloses a hydraulically controlled fan drive system having a certain method of engagement. The hydraulically controlled system includes a housing assembly containing a hydraulic fluid and an engaging circuit. The engaging circuit includes a pitot tube coupled within the housing assembly that receives at least a portion of the hydraulic fluid. An engaging circuit engages the housing assembly to a fan shaft in response to supply of the hydraulic fluid from the pitot tube.
One of several advantages of the above-stated clutch mechanism is that it converts fluid velocity into pressure through use of the pitot tube to generate normal force for engagement purposes. In so doing, the clutch mechanism provides variable engagement via internal hydraulic pressure control. The pitot tube provides an inexpensive pressure supply source that requires minimum space within the fan drive system. To disengage the friction clutch, a fluid controller adjusts the static pressure received by the piston by controlling fluid flow through a controller branch across the main center channel, via the return channel, whereafter the fluid returns to the reservoir. The fluid controller acts essentially as an ON/OFF mechanism to relieve or not relieve the static pressure, thereby either engaging or disengaging the clutch mechanism. The clutch mechanism does not provide a mid-range static pressure level within the pitot tube that can be used to create a partially engaged clutch output based on a viscous fluid type clutching mechanism, thereby allowing more precise control of cooling capabilities within the cooling system.
It is thus highly desirable to create a robust fan drive system that can be used in a wide variety fan clutch applications that is capable of controlling static pressures within the pitot tubes at any given pulley speed and pulley ratio to create mid level speed control.